raquo / xstream.scala

Scala.js interface to XStream.js, a simple functional reactive stream library

Version Matrix


XStream.scala is a Scala.js interface to XStream.js, an extremely intuitive, small, and fast functional reactive stream library for JavaScript.

XStream is notably used in Cycle.js, an honestly functional and reactive JavaScript framework.

"com.raquo.xstream" %%% "xstream" % "0.3.1"


  • Lightweight, type-safe wrapper over XStream.js – native Scala types making use of value classes to avoid extraneous allocations at runtime.
  • Convenient extension methods to work with streams of tuples and streams of streams.

Why XStream?

It's simple and lightweight. It makes the most sense when the developer does not typically subscribe to streams manually. See these posts from the creator of XStream.js:


See XStream.js docs. XStream.scala retains original XStream.js classes and method names with only a few deviations that are easily discoverable with code autocompletion or (less conveniently) by looking in the code.

Notably, class Stream is named XStream in my interface to avoid the inconvenience of sharing the name with scala.collection.immutable.Stream.

When mapping over a stream of tuples, you can use map methods from TupleStream<N> implicit classes to avoid the need for boilerplate or partial functions.

Per XStream.js docs, you should normally avoid using shamefullySend* methods, so in order to use them you need to explicitly create an instance of ShamefulStream. It is a value class, so normally the instantiation does not actually happen at runtime, thus there is no runtime penalty to it.

You should not need to use RawXStream directly, use the XStream object instead.


In XStream.js/Cycle.js, a common convention is to name stream variables with trailing dollar signs, e.g. response$ for a stream of responses. In Scala.js this naming sometimes causes a weird compilation error. I'm not sure why exactly, but I think it might have something to do with Scala's or Scala.js' internal representation making use of trailing dollar signs as well. I will try to figure this out some day, for now I personally prepend $ instead, e.g. $response for a stream of responses. Although if you use jQuery maybe go for a responseStream instead to avoid the confusion.



  • Add tests (XStream has its own tests, but it we should verify this interface as well)
  • I'm not convinced of XStream being actually covariant in T. imitate method seems to violate it, but its use case is also extremely limited to creating proxies. Find a compromise.


  • Add interfaces for XStream extras
  • XStream.of should accept multiple params


  • Document basic usage and scala-specific things (e.g. tuple streams, shameful streams)
  • Copy method documentation from original xstream
  • Add more TupleStreamN types
  • Add MemoryTupleStreamN convenience types


  • Update to latest version of XStream
  • Figure out how versioning should work for this project (match XStream.js version?)

Reporting Bugs

  • If you found a bug, please open a Github issue describing actual and expected behavior.
  • Please make sure that the bug is related to my Scala interface, not the underlying XStream.js library.

My Related Projects

  • Scala DOM Types – Type definitions that my projects use for all the HTML tags, attributes, properties, and styles
  • Scala DOM Builder – Low-level Scala & Scala.js library for building and manipulating DOM trees
  • Scala DOM TestUtils – Test that your Javascript DOM nodes match your expectations
  • Laminar – Reactive UI library based on Scala DOM Builder
  • Snabbdom.scala – Scala.js interface to a popular JS virtual DOM library
  • Cycle.scala – Scala.js interface to a popular JS functional reactive library


Nikita Gazarov – raquo.com

License and Credits

XStream.scala is provided under the MIT license.

XStream.scala is a Scala.js interface to XStream.js, which we use under the terms of its MIT license.